The invention concerns novel polybenzoxazole precursors and resist solutions containing these precursors.
In microelectronics, highly heat resistant polymers are required as protective and insulating layers. These kinds of polymers can be used as a dielectric between two metal planes, e.g. in multi-chip modules and memory and logic chips, or as a buffer coat between the chip and its outline. Some of these polymers, for example precursors of aromatic polyimides (PI) and polybenzoxazoles, are readily soluble in organic solvents, have good film-forming properties and can be applied to the electronic components by means of low-cost centrifugal technique. The precursors are then cyclized through heat treatment, i.e. converted into the corresponding polyimide and polybenzoxazole polymer, and thereby acquire their final properties. 
Cyclization results in disappearance of the polar, hydrophilic groups of the PBO precursor (OH, NH and CO), which would otherwise exert a negative influence on the dielectric properties and the water uptake. This is, for example, a significant advantage of polybenzoxazoles in comparison with polyimides and, in particular, in comparison with hydroxypolyimides. However, the cyclization is important not only for the good dielectric properties and the low water uptake of the final product, but also for its high temperature stability.
The specifications for the cyclized final product are very demanding. Thus, for example, in addition to the dielectric constant being as low as possible, the thermal stability must be high.
Polyimides and polybenzoxazoles have the following advantages over many other high-temperature-stable polymers:
In contrast to the cyclized final product, they can be applied to a substrate as a soluble precursor and then cyclized on the substrate, whereby the solubility and therefore the sensitivity towards solvents and other process chemicals is considerably reduced. The processing of pre-cyclized polybenzoxazoles, for example, is difficult for this reason.
Through addition of appropriate photoactive components to the precursors, photosensitive compositions can be produced which enable low-cost direct structuring of the dielectric. Polybenzoxazoles have the further advantage over polyimides that they can be structured in positive mode and developed in aqueous alkali (cf. EP-PS 0 023 662, EP-PS 0 264 678 and EP-PS 0 291 779). For this purpose the PBO precursors used must be soluble inxe2x80x94preferably metal ion freexe2x80x94alkaline developer.
Benzocyclobutene (BCB), which is processed in a similar way and can be negatively structured, has a significantly lower temperature stability than polyimide and polybenzoxazole.
When using polymers of the above kind as a dielectric between metallic conductors it is very important that the metal does not diffuse through the dielectric at elevated temperatures, i.e. at temperatures greater than 300xc2x0 C. However, many metals, especially aluminumxe2x80x94currently the most commonly used metalxe2x80x94do diffuse through the dielectric at high temperatures. For this reason the metal is provided with a barrier coat, e.g. of titanium nitride or a combination of titanium and titanium nitride, which prevents the diffusion of the metal into the dielectric. However, the use of an additional layer demands a considerably higher outlay of costs and time.
Polybenzoxazole precursors which can be cyclized on substrates and which exhibit good temperature stability are known, for example, from EP-PS 0 023 662, EP-PS 0 264 678, EP-PS 0 291 779, EP-PS 0 905 169, EP-OS 0 905 170 and DE-PS 37 16 629. But there is no information about the diffusion of metals into the polymers prepared from these after cyclization on a substrate (cf. EP-PS 0 264 678 and EP-OS 0 317 942).
It is accordingly an object of the invention to provide polybenzoxazole precursors that overcome the above-mentioned disadvantages of the prior art materials, which can be processed by means of the centrifugal technique, which can be cyclized to polybenzoxazoles on substrates without difficulty, and which after cyclization to polybenzoxazoles exhibit a high temperature stability. In particular, these precursors and the polybenzoxazoles prepared from them should possess high resistance against the diffusion of metals.
With the foregoing and other objectives in view there are provided, according to the invention, polybenzoxazole precursors containing one of the following partial structures: 
wherein each of A1 to A7 is a univalent substituent independently selected from the group consisting of H, F, CH3, CF3, OCH3 and OCF3;
T is a residue selected from the group consisting of 
wherein each of A8 to A21 is a univalent substituent independently selected from the group consisting of H, F, CH3, CF3, OCH3 and OCF3; 
wherein X is selected from the group consisting of xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94C(OCH3)2xe2x80x94, xe2x80x94C(OCF3)2xe2x80x94, xe2x80x94C(CH3)(C6H5)xe2x80x94xe2x80x94C(C6H5)2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94(NH)xe2x80x94, xe2x80x94(Nxe2x80x94CH3)xe2x80x94 and xe2x80x94(Nxe2x80x94C6H5)xe2x80x94; 
wherein M is selected from the group consisting of residues represented by formulas 10-14 
in which Q is selected from the group consisting of Cxe2x80x94H, Cxe2x80x94F, Cxe2x80x94CH3, Cxe2x80x94CF3, Cxe2x80x94OCH3, Cxe2x80x94OCF3 and N,
and residues represented by formulas 15-34 shown below, in which Q is defined as above, provided that at least one Q signifies N and a maximum of two N atoms are present per ring;
and residues represented by formulas 15-34 
wherein Q is defined as above, provided that at least one Q signifies N and a maximum of two N atoms are present per ring.
Dielectrics prepared from polybenzoxazole precursors of this kind exhibit, in particular, a significantly reduced aluminum diffusion. As a result, an additional barrier layer is superfluous.
The metal diffusion can then be reduced still further if acetylene groups are integrated into the precursor molecules. These acetylene groups can be present in the main chain of the precursor molecules, as a terminal group of a precursor molecule, or as a side chain of a precursor molecule.
Preferably the acetylene group is contained in one of the following compounds: 
With the foregoing and other objectives in view there are also provided, according to the invention processes for preparing polybenzoxazole precursors with the above structure by a self-condensation reaction of an appropriate o-aminophenolcarboxylic acid and by reaction of appropriate bis-o-aminophenols or o-aminophenolcarboxylic acidsxe2x80x94whereby in each case one or several of these compounds can be usedxe2x80x94with one or more suitable dicarboxylic acid compounds such as dicarboxylic acids and dicarboxylic acid derivatives, in particular active esters and chlorides. Active esters of dicarboxylic acids include, for example, the methyl, t-butyl, and phenyl esters. For this purpose the bis-o-aminophenol or o-aminophenolcarboxvlic acid and the dicarboxylic acid or dicarboxylic acid derivative, when present, are reacted together, suitably in an organic solvent at a temperature from xe2x88x9220 to 150xc2x0 C., and the resulting polymer precipitated by adding the reaction mixture dropwise to a suitable precipitating agent. The precipitated polymer can be used immediately after being filtered off and dried. Before the precipitation of the polymer, its terminal amino groups can be protected, i.e. blocked, using a dicarboxylic acid anhydride or a monocarboxylic acid chloride. The o-aminophenolcarboxylic acids can also be reacted alone, i.e. without a dicarboxylic acid.
The bis-o-aminophenols and o-aminophenolcarboxylic acids used for preparation of the polybenzoxazole precursors according to the invention have the following respective structures: 
in which A1-A3, A4-A7, and T are as defined above.
These compounds are the subject of German patent application submitted at the same time: File No.
xe2x80x9cBis-o-aminophenols and o-aminophenolcarboxylic acidsxe2x80x9d (GR 00 P 8529 DE).
The designations xe2x80x9cA1-A3xe2x80x9d, xe2x80x9cA4-A7xe2x80x9d, xe2x80x9cA8-A10xe2x80x9d, xe2x80x9cA11-A14xe2x80x9d, xe2x80x9cA15-A17xe2x80x9d and xe2x80x9cA18-A21xe2x80x9d in the structural formulae mean that the phenyl groups or cyclic structures carry respectively the residues A1, A2 and A3, or A4, A5, A6 and A7, or A8, A9 and A10, or A11, A12, A13 and A14, or A15, A16 and A17, or A18, A19, A20 and A21.
Examples of this kind of bis-o-aminophenol and o-aminophenolcarboxylic acid are:
9,9-bis(4-[(4-amino-3-hydroxy)phenoxy]phenyl)fluorene 
9,10-bis(4-[(4-amino-3-hydroxy)phenoxy]phenyl)anthracene 
6,6xe2x80x2-bis[(4-amino-3-hydroxy)phenoxy]-5,5xe2x80x2-dimethyl-3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis(indane) 
4,7-bis[(4-amino-3-hydroxy)phenoxy]-1,10-phenanthroline 
9-(4[(4-amino-3-hydroxy)phenoxy]phenyl)-9-(4[(2-trifluoramethyl-4-carboxy)phenoxy]phenyl) fluorene 
9-(4-[(4-amino-3-hydroxy)phenoxy]phenyl)-10-(4-[(4-carboxy)phenoxy]phenyl) anthracene 
6-[(4-amino-3-hydroxy)phenoxy-6xe2x80x2-[(4-carboxy)phenoxy]-5,5xe2x80x2-dimethyl-3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis(indane) 
4-[(4-amino-3-hydroxy)phenoxy]-7-[(4-carboxy)phenoxy]-1-10-phenanthroline 
Any dicarboxylic acid can be used in preparing a polybenzoxazole precursor according to the invention. Dicarboxylic acids such as 4,4xe2x80x2-oxybisbenzoic acid, 2,2-bis(4-carboxyphenyl)perfluoropropane and isophthalic acid and mixtures thereof are especially suited for the preparation of the precursors. In principle, however, all dicarboxylic acids can be used which have up to three aromatic nuclei.
A monocarboxylic acid can be used in controlled minor amounts as chain terminator and molecular weight regulator in preparing a polybenzoxazole precursor according to the invention. Aromatic monocarboxylic acids are preferred.
In a particularly preferred embodiment, dicarboxylic acid used in preparing polybenzoxazole precursor according to the invention includes p,pxe2x80x2-ethinylbis(benzoic acid) represented by above formula 37 to integrate the acetylene group into the main chain. Similarly, dicarboxylic acid used in preparing polybenzoxazole precursor according to the invention includes (2xe2x80x2-phenylethinyl)benzene-1,4-dicarboxylic acid represented by above formula 38 to integrate the acetylene group into a side chain.
When a monocarboxylic acid is used as chain terminator, 2-phenylpropynoic acid represented by above formula 35 and/or p-(2xe2x80x2-phenylethinyl)benzoic acid represented by above formula 36 can be used to integrate the acetylene group into the end group.
If dicarboxylic acid chlorides are used for the polymerization, the use of a basic acid scavenger is advantageous. Any basic scavenger can be used. Preferred acid scavengers of this kind are pyridine, triethylamine, diazabicyclooctane and polyvinyl pyridine. However, other basic acid scavengers can also be used, whereby especially preferred are those which are readily soluble both in the solvent used for the synthesisxe2x80x94for example N-methyl pyrrolidonexe2x80x94and in water/alcohol mixtures (precipitating agent), and also those which are completely insoluble in the solvent, for example cross-linked polyvinyl pyridine.
Especially suited as solvents for the polymer synthesis are dimethylacetamide, xcex3-butyrolactone and N-methylpyrrolidone. In principle, however, any solvent can be used in which the starting materials are readily soluble. Especially suitable precipitating agents are water and mixtures of water with different alcohols, for example ethanol and isopropanol.
The polymer precursors according to the invention are readily soluble in many organic solvents, such as acetone, cyclohexanone, diethylene mono- and diethyl ether, N-methylpyrrolidone, xcex3-butyrolactone, ethyl lactate, tetrahydrofuran and ethyl acetate, as well as in aqueous alkaline metal-ion-free developers, and can be processed without problems by means of centrifugal technique. After cyclization on the substrate the polybenzoxazoles obtained as the product exhibit considerably reduced aluminum diffusionxe2x80x94in comparison with materials of prior artxe2x80x94and enhanced high temperature stability.
The polymer precursors according to the invention are compatible with diazoketones and are therefore advantageously suited for photoresist solutions whichxe2x80x94dissolved in a solventxe2x80x94contain a polybenzoxazole precursor and a diazoketone based photoactive component. Surprisingly, such photoactive compositions exhibit high resolution and a very good film quality. Preferred diazoketones include diazoquinones such as naphthoquinonediazide-5-sulfonic acid.
In the photoresist solutions the ratio by weight of polybenzoxazole precursor to diazoquinone is advantageously in the range from 1:20 to 20:1, preferably from 1:10 to 10:1.
The polybenzoxazoles corresponding to the polybenzoxazole polymer precursors according to the invention include the following partial structures: 
wherein A1 to A7 and T have the meanings given above.
In the polybenzoxazole polymer precursors according to the invention, and in the corresponding polybenzoxazoles, the indicated partial structures according to the invention are linked together and to residues of such dicarboxylic acids and monocarboxylic acids as are used in their preparation.
Although the invention is illustrated and described herein as embodied in polybenzoxazole precursors and processes for their preparation and utilization, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The practice and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description in greater detail on the basis of exemplary embodiments: